The present invention relates to a method and apparatus for heating a pre-coated plate of steel.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Production of hot formed structural parts is based on plastic formation of mostly flat semi-finished products. Unlike cold forming at room temperature, the preceding heating of metallic semi-finished products in particular prevents the presence of unwanted solidification and decreased ductility in the forming zone. In addition, heating facilitates a targeted change in shape of the semi-finished product because the reduced strength of the used material, when heated, substantially avoids the presence of possible shearing or cleavage fracture.
Steel plates provide in particular in the automobile industry the basis for manufacturing body or structural parts. Besides corrosion protection, ecological and economic considerations dictate an increasing need for high-strength structural parts which have a beneficial ratio of strength to weight. Mechanical resistibility can be increased by hardening the material through heating and subsequent rapid quenching to cause a change in position of the carbon atoms in the metal lattice. This positional change begins when the austenitization temperature is reached, with subsequent quenching producing a martensitic hardened microstructure to thereby significantly increase the strength of the formed structural part. The required cooldown rate is hereby dependent on the respectively used alloy.
When the use of thin-walled plates of steel is involved, form or press hardening has established itself as an economical process to hot form metal sheets. The heated plate is placed in a forming tool to undergo the forming process and hardening as it is quenched. To prevent decarburization and oxidation of steel during heating, the latter is carried out in the presence of a controlled atmosphere, for example nitrogen. Heating may, however, also take place in the presence of ambient air so long as the plate has been suitably coated prior to undergoing the heating process, for example with a coating of aluminum or aluminum alloy such as aluminum and silicon.
The use of oxides as passive corrosion protection on the surface of metals is generally known. In order to obtain the positive properties of oxides also on a surface of a coating, exposure to atmospheric oxygen is desired during heating. However, nitrogen naturally contained in the ambient air forms together with aluminum or an alloy of aluminum and silicon of a coating very hard deposits which adhere to the forming tool. For the surface quality of structural parts being produced to not deteriorate, the tools have to be cleaned. This causes not only stoppage and set-up and maintenance works, but a removal of hard deposits requires also grinding of the forming tool zones so that wear is significantly increased. As the furnace atmosphere is heated, the contained oxygen proportion is decreased at least in some areas so that the formation of the desired oxide layer on the coating is limited. As a result, the oxide layer is unable to fully develop in order to counteract the adherence of the coating on the forming tool so that added deposits are encountered.
In addition, as the aluminum oxide layer is not fully developed and partly detaches, dust is increasingly formed, causing increased wear through abrasion of the guided and/or bearing-mounted components of the forming tool. Thus, guides of slides or brakes of the forming tool for example are subject to increased wear. Due to the uncontrolled atmosphere inside the furnace, a respective water fraction in the form of water vapor is encountered as a result of an exchange with the ambient air. Water is broken down by thermal stress inside the furnace and leads to an increased proportion of hydrogen which can cause hydrogen embrittlement of the steel. For economical reasons, the furnace is built with small openings for charging and discharging so that only a small fraction of atmospheric oxygen is able to migrate into the furnace and thus again the formation of an advantageous oxide layer on the coating is limited.
It would therefore be desirable and advantageous to provide an improved method and apparatus for heating a pre-coated plate of steel to obviate prior art shortcomings and to reduce wear of a forming tools as a result of deposits and to reduce abrasion while yet to enable the formation of a sufficient oxidation of the coating and to reduce the risk of hydrogen embrittlement in an economic manner.